Generally, for a construction machine such as a hydraulic excavator or a hydraulic crane, a working mechanism for performing excavating or loading, for example, is liftably provided. As such a working mechanism, a hydraulic excavator that includes a front device such as a boom or an arm is well known (e.g., see Japanese Patent Laid-Open No. 2001-81810).
Consider the boom of a hydraulic excavator as an example of a front device according to this first prior art. This boom includes a box member, which is formed as a hollow structure having the shape of a box by welding together an upper flange, a lower flange, a left web and a right web, and a boss which is located on the base end of the box member, and is liftably coupled with the vehicle body of a hydraulic excavator.
In this case, the boss is formed by a cylinder which is made of a metallic material having a high strength, and the ends of the upper flange, the lower flange, the left web and the right web of the box member are welded at the external surface. Further, a bracket which the arm of the working mechanism is rotatably connected is provided at the distal end of the box member.
Furthermore, according to the first prior art, various reinforcement structures are provided to obtain the strength of the hollow box member. In this case, in the first prior art, for example, a concave rib is formed at the middle position in the longitudinal direction of the left web of the boom.
Further, as a second prior art, a structure is well known wherein, for example, the upper flange, the lower flange, the left web and the right web of a box member are formed of thick steel plates, and a blocking plate is provided inside of the box member to block the internal space at the middle position in the longitudinal direction (e.g., Japanese Patent Laid-Open No. Sho 53-31539).
In the first prior art mentioned above, since the boss which has a high strength is welded to the upper flange, the lower flange, the left web, the right web which have lower strengths, there is a demand at this stage in the designing of a front device that the strength be obtained at the position whereat these two materials which have different strengths are welded together.
However, according to the structure of the first prior art, the rib is located in the middle in the longitudinal direction of the left web of the boom. The rib can increase the flexural strength of the left web, but there is a limit to greatly increasing the strength of the box member as a whole. Thus, there is a problem that it is difficult for a satisfactory strength to be obtained near the boss.
Further, according to the structure of the second prior art, the upper flange, the lower flange, the left web and the right web of the box member are formed of thick steel plates and the blocking plate is provided inside. However, when thick steel plates are simply employed, the weight of the boom is increased, which causes problems, i.e., an increase in the size of an actuator that drives the boom and results in the reduction of the operation efficiency. In addition, even when the blocking plate is provided inside of the box member, the strength may be insufficient near the boss because this blocking plate is located separate from the boss.